All kinds of vegetation is rapidly formed on the bottom of boat, which slows down the speed of the boat and increases the fuel consumption. Soiling is prevented primarily by the use of different toxic paints. The use of toxic paints is being restricted in several countries because of the environmental detriments thereof. This development has given rise to a vigorous need to have as automatic boat washing means as possible. A proper cleaning of the bottom of a boat can be performed by pulling the boat ashore but it is an expensive measure. Boat washing means floating in the water are known in the art, such as means according to patent specification U.S. Pat. No. 4,046,095, U.S. Pat. No. 4,043,286 and U.S. Pat. No. 4,007,701. According to said patents, a boat is pulled with a mechanical conveyor between a washing means located between two pontoons, in which the brushes guided by a complicated mechanism carry out the washing. Such means are complicated and expensive. Also a boat washing means according to Finnish utility model No. 574 is known. In order to operate properly, said means requires four pairs of scrubbing brushes, wherefore the structure is also relatively expensive.